Residents of Wanganui rest homes are waiting an average of 79 days to receive the right care - more than three times longer than anywhere else in the country.
And the Aged Care Association has warned that, until the Whanganui District Health Board acts, vulnerable elderly people are in danger.
But the WDHB has said the figures released by Aged Care are the results of calculating waiting times differently from other DHBs.
The association has released information on the time between a patient receiving a referral and an assessment of their needs. The figures - which include the lowest, the highest, and the average number of days between a referral and an assessment - are for those in rest homes during the quarter ending June 2012.
Elderly in the Bay of Plenty are the best off, with an average waiting time of four days. Whanganui DHB is the longest at 79 days, with the next biggest gap Southern DHB, with an average of 24 days.
Aged Care chief executive Martin Taylor said the Wanganui figures were "horrendous".
"It is certainly a concern for us, because it does impact on the safety of elderly people. If someone is assessed as needing dementia care, they can't wait 79 days - they need that care immediately."
Mr Taylor said to the best of his knowledge there had been no adverse incidents in Wanganui involving dementia patients, but that was probably because rest homes recognised the need for increased care and provided it.
Aged Care believed best practice would see a person assessed within three days of a referral. An acceptable period would be 10 working days.
"Only seven DHBs meet this criteria: Bay Of Plenty, Hutt Valley, Capital & Coast, Auckland, Taranaki and West Coast. Another seven DHBs on average take 15 days to carry out an assessment."
Mr Taylor said Aged Care would continue to monitor waiting times for needs assessments.
"We would like to see a improvement, particularly in Wanganui," he said.
Labour health spokeswoman Annette King said she was shocked to see Wanganui's waiting times.
"I am worried about the neglect of elderly people. Clearly, this is not a big enough priority for the Whanganui DHB."
Mrs King said elderly people were particularly vulnerable and, for many, a wait of 79 days was too long.
"Hips can be broken, people can end up in acute care at great cost to the health system, and people can die.
"I think about my own elderly dad, who is getting close to needing residential care. If it took 79 days to get him assessed, we'd be pretty unhappy."
Whanganui District Health Board allied health manager Louise Allsopp acknowledged that it had longer waiting times for a comprehensive clinical and support assessment than other DHBs last year due to the roll-out of a new needs assessment system.
But she said she was confident that those waiting times were not significantly different from other DHBs.
"We assume that the apparent difference within the information released by the Aged Care Association is because we counted everybody including those receiving short-term support. And secondly, we believe we might have calculated the average waiting time differently to other DHBs in that we excluded from the average everyone who was seen within the first 24 hours."
Mrs Allsopp said if people were waiting in their own home or in hospital to be assessed for long-term residential care, the timeframe was one to four days, with an average of two days.
"People in their own home are most at risk and, therefore, our top priority. People who are placed on the waiting list are those who are already receiving short-term support services or who are being closely monitored by clinical staff or home care agencies," she said.
Mrs Allsopp said home care agencies and clinicians met frequently to discuss any concerns they might have about a specific person.
The number of people receiving a comprehensive clinical and support assessment in the timeframe covered by the figures was 197.